Abstract
The electroless Ni (EN) plating method was employed to metallize the A1N ceramic substrates. The EN-plated A1N substrate was bonded with the Cu foil to form a sandwichlike A1N-EN/Cu/EN-A1N assembly by hot pressing in vacuum with a pressure of 6.5 MPa for 30 min. For the bonding temperature below the Ni-P eutectic temperature of EN at 880°C, the samples were bonded through solid state diffusion. On the other hand, the samples were bonded via a liquid phase media through both wetting and diffusion if the bonding temperature was above 880°C. An optimum adhesion strength around 10 MPa occurred within bonding temperature range 600-700°C. The fracture took place in the EN/Cu interface for samples bonded below 600°C, while fracture took place in the A1N/EN interface above 700°C. The increasing temperature enhanced interdiffusion of Cu and EN to form a strong bond, yet resulted in a large residual thermal stress in the A1N/EN interface. The bonded samples with as-received A1N exhibited higher adhesion strength than those with polished A1N because there existed a residual compression perpendicular to the A1N/EN interface at surface irregularities in the as-received A1N, which resulted in an additional shear strength offset in the adhesion test. The adhesion strength of samples with etched A1N was the highest as compared to those of as-received and polished A1N, although the surface roughness of the etched A1N was the same as that of the as-received one. It is argued that the etched surface of A1N with micro-etched holes provides the anchor sites for interlocking with the EN film, which results in a good mechanical bonding in the joint. However, a mechanical trimming on the edges of bonded samples would damage the joint, and a low adhesion strength is instead observed. © 1994 IEEE